Stretchable pipe (for export to Unity)
I'm not entirely sure what you want, but you can make a "stretchable pipe" in Blender by using Hooks:


Add a cylinder with a fill type of Nothing
Add hook to one end by pressing CtrlH> Hook to new object:



This will add an empty with a Hook modifier that you can control the hooked vertices with.
Add a hook to the other end.


Now you can manipulate the newly created empties to stretch the "pipe".